Measurements and Quantifications for the Lung Nodule Motion Using 4DCBCT Scans Prior to Each SBRT Treatment

Purpose: To measure the lung nodule motion using 4D CBCT, to quantify any potential influences on the motion trajectory, and to derive an analytical approximation for motion trajectory in order to synchronize with external real-time respiratory motion tracking.

Methods: GTV from fast CT and ITV from CT-PET obtained from rigid or deformable co-registration with simulation CT are used for SBRT planning. Pretreatment 4D CBCT with ten phases are automatically registered according to the GTV mask. The nodule motion trajectory is quantified by the center, amplitudes, swiping area, major and minor axes in the principal plane (PP), and the PP normal vector. The setup errors and motion margins are assessed by the trajectory center and amplitude, respectively. Detected motion in the PP is analytically approximated by an ellipse with time parameter and the trajactory shape or axes and orentation may be correlated with the lung volume, nodule location, abdominal compression, and restriction force from the neighboring chest wall or other rigid tissue. Those variables in seventy-two 4D CBCT scans from 34 patients are retrospectively analyzed.

Results: After 4D CBCT-guided setup adjustments of 10±8 mm, the remaining setup errors in any directions increase from 1±1 mm for upper and middle lobes to 1± 2 mm for the lower lobes while the average motion margins in the lateral, longitudinal, and posterior-anterior directions increase from (1, 2, 1) to (1, 6, 1) mm. No significant difference is observed between the left and right lungs and between the groups with and without abdominal compression. Importantly, most motion trajectories can be analytically approximated and its PP is predicable with the local force.

Conclusion: The clinical measurements and detail analysis validate the geometric accuracy and precision of current SBRT and our analytical approximation is potentially useful for synchronization with real-time respiratory motion tracking for future gated SBRT.